You could use a text password checking code
either the one I use or the one that is in AUM5. At the end of disc 1 you could have a panel that appears and gives a password and will instruct the user to load disc 2. When disc two boots a panel will appear that will ask for the password and if you give the right password then the panel disappears and the you can now play on. See your 1st level will load on disc 2 but it will be behind the panel in freeze mode waiting for the password to be typed in. It should work. If you need some password code I have it. GS out.